ILMRoss

After watching tomorrow's system slip through the cracks for central NC because of a ULL getting too amped I'm pretty fine if the modeling consensus is too suppressed/weak this far out edit- been playing golf today then immediately switched gears to watch football at a buddy's place just saw the 12z suite for the first time oh my god

The last thing I’m doing tonight and the first thing I’m doing tomorrow morning is checking out dews upstream of us and how they’re verifying on Iowa state meteogram. I spoke on this yesterday, I think there’s a few more things with storm than usual that could erode our CAD, and if it hangs on/overperforms then It solidifies itself as the dominant forecasting feature for winter storms. One last thing I’ll touch on since I saw some chatter about it, Jan 2000 was both cool, and could also never happen again. Our tools are a bit better today than the TI-89 graphing calculators they were using to run models back then.

The NAM has this reputation because it fired the first warning shots in the late Feb 2015 storm and early Jan 2017 storm. In both those storms RDU was basically center cut in the jackpot by every model except the NAM. Everyone was like, "lol crazy NAM", and was unprepared for the gut punch those storms delivered when sleet/rain began to mix in hard. Since then everyone is pretty petrified to bet against it.

Setting the table with what dews look like on 00z Sunday..... 18z HRRR: 12Z NAM 3km: WRF-ARW (??) WRF-ARW2: Regular NAM: GFS: Going to ask my group chats why the ARWs (research models) have such low dews

In my experience there's like 2 ways to overperform on modeled snowfall and 87 ways to underperform on modeled snowfall.

Some thoughts... - I try to keep a sunny disposition on here because at the end of the day this is a lighthearted forum and not some emergency management network. That colors my thinking and I'd say I have a bias posting about "good things that could happen" and not "failure points". I say this because I've banged the drum about SLP adjustments being too inland for the CAD, models mishandling the transfer that could lead to a more south track, etc and this serves as my "I hope I have not lead anyone astray with false optimism" mea culpa as a rainier picture takes hold. To build on this: - I think the models have a great grasp of what's going to happen. There will be ticks and adjustments and some low stakes twists and surprises but the playbook is out. - Over the last few days, I thought the CAD strength would redirect our SLP further off the coast. That's happened to a certain degree... (no more runs have this thing visiting South of the Border) but I'm unsure what tangible effects this has on the forecast and I thought that sub-freezing temperatures would hang on a little longer as this modification played out. - "The CAD will hang on" is a sound motif in 90% of of situations. I'm beginning to think this is the 10% of cases where that may not hold true. Typically in these scenarios the thing scouring the CAD is screaming 850mb winds. The rate at which the CAD gets eroded is determined by "how well can this model handle the turbulence and eddies between the stable CAD and the screaming 850mb jet". There's micrometeorology and microphysics wizardry going on there and typically the answer to that question is "not very well". However, in this storm we also have crazy lift ahead of a bowling ball upper level low as well as a convection (maybe severe-ish) nudging in from the coast. Our upper air features are typically a lot weaker. This will be a learning experience for everyone, me included, and while I think there's still ample potential for the CAD to hang on, I can easily see why that motif wouldn't hold true with this system and it dissolves much quicker than we're used to. (For clarification: I am not rooting for a strong CAD. My ideal Sunday is seeing a quick hour burst of snow before settling in at a sports bar and eating 1000 wings and watching the Eagles cover and the 49ers win outright and I can't do that without power) As far as tangible forecasting goes: - Shoutout to the mountains. You should get buried unless there's some Miller B voodoo BS going on and the thermals/moisture transport get screwed up (which can happen). Also shoutout to Nashville who is having a fantastic winter. - Sleet/ZR amounts will continue to wobble. Let me bring you through the life of a snowflake through CAD. You fall, melt (cooling the atmosphere) then either refreeze into sleet (releasing latent heat 1000 feet up) or fall as freezing rain (releasing latent heat at the surface. No model is going to be able to nail this down correctly. I also wonder about how latent heating is modeled... Are models assuming everything falling will freeze? In the southern fringes of the ice zone, there are some pretty insane rates being modeled for a winter storm and a lot of that precip will be going down the drain. Are the models assuming that's going to freeze and the latent heat equation is treating it as such? I don't know. - I'm not making a snowmap, the market is saturated and I'm no clout chaser.

NAMs great for a lot of things; thermals, soundings (for both svr and winter), convection is modeled better. Higher resolution always helps. And it’s generally good to have as many models in your camp as possible. If you watch a a lot of football you may occasionally see an offensive lineman go down and the announcers will say “ohh man they’re going to miiiiiissssssss him”. And if he’s on your team, you may think “ah ok well it’s not the quarterback” but deep down you do get worried. Basically how I feel when the NAM or something similar is souring. Hopefully that analogy made sense.

Still a long way to go with this run. But my feeling is that if you are in a marginal area, you’re not going to enjoy the 00z NAM.

Yup nice little finger of before the storm precip im definitely keeping an eye on it

If you are already comparing the NAM right now from the 18z, you need to relax. Step away from the computer. I also thought the shortwave looked a little bit weaker than the 18z at hour 15.

C Is there a wedge boundary here that the low pressures are trying to transfer/bend around? Kind of hard to tell.

Probably depends on which RDU red tagger you ask. I think that the CAD maintains its Integrity a little more and it's predominately sleet/zr/711 slushie after a quick 1-2 inch burst for now. I don't think we'll get above freezing. Eyewall just said rain/zr split which is probably just as likely as my thinking. Think of it like this:

I just flipped through 10m wind speeds through all of the globals and meso model at 78 and a few things jumped out: - every mesoscale model kept the wedge pinned on the coast and had the low in south geogia-ish - every global (except the canadian) had eroded the wedge further inland and where the wedge had eroded is generally where the LP followed (with Euro obviously showing the most erosion of the wedge) take with this what you will.

Feeling good but not great after that run. Morning's 00z appears to be more and more of a warm blip and this run assuaged some of my concerns. That shortwave diving in behind this thing though just continues to be a massive PITA. That's the feature that's causing our heavy snow axis to take a 45 degree angle (along I-81) instead of a 30 degree angle (up I-85). At this point I don't think that thing's letting go.

good grief, i think this is a pretty bizarre thing to greenlight

BTW, if you want the "pie in the sky, 5% chance of it happening" scenario, its the ULL actually reaching the coast instead of tracking just inland. Things would go boom. (There's a reason the more offshore solutions in the GEFs solutions are in the 70-80mb range, and not in the 90s)

I noticed something in the NAM suite that I thought was very promising for our general low evolution, and it's indicative of why I like to wait for the convection allowing models before I really make broad remarks on the low pressure track and how it transfers to the coast. The low transfer is going to occur along the ribbon of low level vorticity that forms off the coast along the wedge front. It's really easy to see this on the 3km NAM at hour 60: So, we have transfer area. Transfer portal if you will. No other model really has this depicted well. 6z GFS at 66: 12Z NAM (not hi res) which is a tough better: Seems like a pretty small detail but it's important because if you get a sharp boundary you'll get convection/storms to form along it. Thunderstorms will take that low level vorticity and stretch it, inducing cyclogenesis and stretching that air column. How important is this feature? Elevated convection, and models not handling the ramifications well of that convection well, is how we ended up with the debacle that was Jan 2000 (I'm saying that as an example, in the year of our lord 2022 that's not happening again please dont twist my words.) Well, the 3KM NAM has some of that action at hour 60. My entire point here is that I still think the models aren't handling the cyclogenesis along the coast well and we're still leagues away from figuring out how the transfer will work. I think there's still room for some corrections further off the coast. Or maybe this is just weenie wishcasting. There's too much football on on Sunday for my power to go out so I'm going to manifest better solutions by making long posts on weather forums.

if looking at the hr 84 NAM is hazardous for your health then this run is probably going to send half of this board to the ER

Jeez man who pissed in the euro’s coffee. Didn’t expect to wake up to such an abjectly bad run.

This, to me, is a model that’s saying “trust me guys, I know what im doing here”. I know a model is sniffing something out when the isobars in North Carolina look like the Map app when I drove on the blue ridge parkway a few years ago. (If this is your first rodeo, this is sarcasm, I think whatever surface map the GFS is putting down is pretty bogus. No surface map should look this arbitrarily wavy. Nice tick to the SE tho!)

Whoa, the ICON pulls the 500z ULL through the Carolinas... breath of fresh air after these models wanted to bring it up the spine of the apps. The 00z ICON, I should mention, is warmer on what looks like every level. edit it's a little warmer but not on "every level" didn't look everywhere i'm multitasking between model watching and dishes

It's not so much a dry bias as much as is it... not quite sure how to explain. A precip shield looking like a circle when it should look like an oval. Phantom frontogenesis bands that come and go. I don't have a scientific word other than weird. Things like that when everything else will look fine. When i first became an enthusiast the NAM was known for throwing out ridiculous qpf and its not that.

I thought the most consequential thing that the NAM with the "new data" showed was a wholesale shift of our shortwave 50 miles to the east as early as 36 hours into the run. I like using the NAM as an appetizer model for a lot of reasons and one is that I think it can tip the hand of the rest of the models on what the trend de jour of the suite is going to be. I think we're cooking with propane if more models show this shift east with our energy. I will say, one thing i don't use the regular NAM for is precipitation depiction. I've always thought it has looked a bit 'off' with its precipitation depiction, dry areas where I'd expect more precip, etc. I think it's fun to look at but I don't hang my hat on it- I lean on the 3km a lot more for that in my internal calculus.

Man that was just an unambiguously terrific GFS run. Weaker and faster with that energy diving in and it just made all the flippin difference. Solid 150 mile shift SE! Reel in like 2 more of those kind of shifts and a whole lot more of this board will look good. I will preach patience, let's give it the 00z suite and 12z suite tomorrow to make sure this run is not some herky jerky blip (although as previously noted, this would continue the NW trend reversal first seen on the 12z)

Want to emphasize that the big fat "L" doesn't tell the full story: see below. Despite the southerly position, there's still a low pressure area extending to Memphis. I make this point to basically say don't get too carried away by the Ls near the optimal positions. By perfect getting in the way of good, I was talking about sticking this pre-made graphic on socials and on a show instead of handmaking a graphic which is time consuming. But I totally see your point now and I agree. It's been a while since I've used it but I think that the software WRAL uses has rounding options and I'm wondering why they don't use it- may be a consultant thing.